Controller for a switched mode power supply (SMPS), a SMPS, and a method of controlling a SMPS

ABSTRACT

According to an example embodiment, a controller for a Switched Mode Power Supply having opto-coupler-based feedback from secondary to primary side, is disclosed, in which the optocoupler current varies inversely with the output voltage over a voltage control range. The converter is thereby enabled to consume less power than do conventional converters, when in lower-power standby mode. Also disclosed are low-voltage startup and over-voltage protection arrangements combined with such a controller. Corresponding methods are also disclosed.

This application claims the priority under 35 U.S.C. §119 of Europeanpatent application no. 09176055.3, filed on Nov. 16, 2009, the contentsof which are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to switched mode power supplies (SMPSs), tocontrollers therefore and to methods of controlling SMPSs.

BACKGROUND OF THE INVENTION

Switched mode power converters, in particular but not limited to AC-DCconverters, typically employ feedback from the secondary to the primaryside in order to control the power of the converter. For manyapplications, however, electrical isolation is required between the twosides, typically in order to protect the user on the secondary side fromrelatively high voltages on the primary side (such as mains voltages).For such applications, an optocoupler is a preferred means to providethe feedback across the isolation, since feedback information istransferred from the electrical domain to an optical domain and back.

In known feedback systems, the optocoupler is used to transfer an errorsignal (Vout−Voutref), indicative of the difference between the actualoutput voltage (Vout) and the desired output voltage (Voutref). Thefeedback current on the output side of the optocoupler (that is to sayon the primary side of the converter), increases typically linearly withthe error signal, as shown schematically in FIG. 1, which shows a plotof the output current (Iopto) 10 from the optocoupler, against theconverter output voltage (Vout). The feedback current Iopto is typicallydirectly related to the input current (Ioptin) driving the optocoupler(that is to say, on the secondary side of the optocoupler)—for instance,the feedback current Iopto may be directly proportional to the inputcurrent (Ioptin), through a gain factor g. In some implementations, forconverter voltages below or equal to Voutref, the feedback current iszero, whilst above Voutref the current increases linearly with the errorsignal (Vout−Voutref). Further, the error loop may includefrequency-dependant elements for implementing integrating and/ordifferentiating actions.

European patent application EPA1,648080 discloses a controller in whicha feedback signal rises in inverse proportion to the output voltage.

Under low-power, or no load, conditions the current required by theoptocoupler to feedback the error signal can be relatively high. Inparticular to increase converter efficiencies at low load, there is anongoing desire to reduce the power consumption in the feedbackmechanism.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda controller configured to control a switched mode power converter, theswitched mode power converter having a primary side, a secondary sideisolated therefrom and for providing an output voltage, and anoptocoupler therebetween for providing feedback information to theprimary side from the secondary side and drawing, in use, a feedbackcurrent from the secondary side, wherein the feedback information isdependant on the output voltage and the controller is configured toprovide that the feedback current monotonically decreases withincreasing output voltage between a threshold voltage and at least anupper bound. Of course, it will be appreciated that a monotonicallydecreasing function may be flat, or invariant, over part of its domain.Thus the optocoupler may consume less power, in a standby mode, than inconventional arrangements.

The controller is configured such that the feedback current is invariantwith the output voltage between the threshold voltage and a referencevoltage (Voutref), which reference voltage is higher than the thresholdvoltage. In these embodiments, the feedback current falls only over anupper part of the range of output-voltage between the threshold voltageand the upper bound. In other embodiments, the feedback current fallsover the lower part of the range, but across this lower part of therange, the feedback current is still sufficiently high to requestmaximum output power. In some embodiments the controller is configuredsuch that the feedback current either is inversely proportional to theoutput voltage above the reference voltage (Voutref) or decreaseslinearly with increasing output voltage above the reference voltage(Voutref).

In embodiments, the controller is configured to receive a signal(Voutprim) from the primary side indicative the output voltage (Vout),and wherein the controller is further configured to override thefeedback information from the optocoupler with the signal (Voutprim)when the output voltage is less than the threshold voltage.

In embodiments the controller is configured to provide an over-voltagefeedback current in the case that the output voltage is above the upperbound.

In some embodiments the controller further comprises a timer fortriggering the over-voltage feedback current only after a predeterminedtime has elapsed.

In embodiments the controller is configured to provide a minimumfeedback current, in the case that the output voltage is above the upperbound. In embodiments the controller is configured to override thefeedback information from the optocoupler in the case that the feedbackinformation is less than a predetermined threshold current which is lessthan the minimum feedback current.

According to another aspect of the present invention, there is provideda switched mode power converter comprising a controller mentioned above.

According to another aspect of the present invention, there is provideda method of controlling a switched mode power converter having a primaryside, a secondary side isolated therefrom and for providing an outputvoltage, and an optocoupler therebetween for providing feedbackinformation and drawing, in use, a feedback current from the secondaryside, the method comprising providing feedback information to theprimary side from the secondary side dependant on the output voltage andgenerating the feedback current such that it monotonically decreaseswith increasing output voltage between a threshold voltage and at leastan upper bound.

The feedback current is invariant with the output voltage between thethreshold voltage and a reference voltage (Voutref). Thus, the feedbackcurrent may have, in part, a flat profile, which does not change as theoutput voltage changes.

In embodiments the feedback current either varies in inverse proportionto the output voltage above the reference voltage (Voutref) or decreaseslinearly with increasing output voltage above the reference voltage(Voutref).

In embodiments of the invention, the method further comprises measuringa signal (Voutprim) on the primary side indicative the output voltage(Vout), and overriding the feedback information from the optocouplerwith the signal in the case that the output voltage is less than thethreshold voltage. In embodiments the controller provides anover-voltage feedback current in the case that the output voltage isabove the upper bound. In embodiments the over-voltage feedback currentis triggered only once the output voltage has been above the upper boundfor a predetermined period.

In embodiments the feedback current is set to a minimum value (10 f, inthe case that the output voltage is above the upper bound. Inembodiments the controller overrides the feedback information from theoptocoupler, in the case that the feedback information is less than apredetermined threshold value which is less than the minimum currentvalue (10 f).

According to a further aspect of the present invention, there isprovided a computer program, which when run on a computer, causes thecomputer to configure a controller as described above, or to operate amethod as described above.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which:

FIG. 1 shows a graph of optocoupler current against output voltageaccording to known converters;

FIG. 2 shows a graph of optocoupler current against output voltageaccording to embodiments of the invention;

FIG. 3 shows a graph of optocoupler current against output voltage,including under-voltage override according to an embodiment of theinvention;

FIG. 4 shows a block diagram of a controller according to an embodimentof the invention;

FIG. 5 shows a graph of optocoupler current against output voltage,including under-voltage override and overvoltage protection, accordingto another embodiment of the invention;

FIG. 6 shows a block diagram of a controller operable according to thegraph of FIG. 5;

FIG. 7 shows a graph of optocoupler current against output voltageaccording to a further embodiments of the invention; and

FIG. 8 shows a block diagram of a controller operable according to thegraph of FIG. 7.

It should be noted that the figure are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these figure havebeen shown exaggerated or reduced in size, for the sake of clarity andconvenience in the drawings. The same reference signs are generally usedto refer to corresponding or similar feature in modified and differentembodiments

DETAILED DESCRIPTION OF EMBODIMENTS

As shown in FIG. 1, conventional arrangements of the feedback circuitryprovides that a higher value of the output voltage results in a highervalue of the optocoupler output current (hereinafter also referred to asthe feedback current); the high opto-current is evaluated by thecontroller to be a request for lower power, and the converter iscontrolled in order to reduce the output power. Conversely, a lowfeedback current is evaluated by the controller to be a request forhigher power, so the converter is adjusted in order to increase theoutput power. One important reason to choose this arrangement is toensure reliable start up: at a low output voltage, the secondary circuitis not supplied with a sufficiently high supply voltage to drive theoptocoupler, resulting in zero feedback current. As zero feedbackcurrent is a low current, this is interpreted by the controller as arequest for higher power, so the power is increased—resulting inreliable start-up.

However, this can result in undesirable power consumption during standbymode, when the load is extremely low. Standby mode can exist forsignificant periods for systems such as power adaptors. In standby mode,the output voltage remains high, even though only low power is drawn.Due to the output voltage being maintained during this operation, theopto-coupler continues to draw high input current, which can result in apower consumption on the secondary side typically of the order 50 to 100mW. Moreover, since this power has to be supplied by the primary side,there is a further power loss due to the less-than-unity conversionefficiency of any converter; thus, the situation is even worse, and theoptocoupler's drain on the input power can be typically 60 to 200 mW.

In order to reduce the loss due to the optocoupler under no load, orstandby, conditions, it is thus desirable to “invert” the operation ofthe optocoupler, as shown figuratively in FIG. 2; then, for increasingoutput voltage (Vout) above the reference voltage (Voutref), the currentdrawn by the optocoupler decreases. FIG. 2 shows the optocoupler current10 against output voltage according to embodiments of the invention. Forvoltages greater than the reference voltage Voutref the current 10 adecreases with increasing voltage. In the embodiment shown, the decreaseis linear with the error (Vout−Voutref); however, other relationshipssuch as quadratic or reciprocal could also be used. In general terms,provided the derivative d(Iopto)/d(Vout) is negative, any relationshipcould be used. Moreover, frequency-dependant components, such ascapacitors, may be used in the feed-back loop, and the decreasingfunction may then be made to depend either in addition, oralternatively, on either a time-integral and/or a differential of(Vout−Voutref).

For voltages less than the reference voltage, the current 10 b is flat,or invariant with voltage, at a nominal first maximum level. Inpractice, the current need not be flat for voltages lower than thereference voltage; it is sufficient merely that the current issufficiently high to require that the SMPS provides the maximum desiredoutput current at the regulated output voltage.

Thus according to this arrangement, the gain of the feedback comprisingoptocoupler is negative, rather than positive according to conventionalarrangements. From one viewpoint, then, it may be considered that thefeedback, or more accurately of the gain of the feedback, has invertedpolarity.

With this arrangement, a demand from the secondary side for more power,that is to say a low value of Vout, results in an increased feedbackcurrent. In standby mode, the output voltage Vout is high, which resultsin a low optocoupler current: this significantly reduces the powerconsumption in standby mode.

However, this approach gives rise to a significant problem duringstart-up and in over-current situations. An over-current situation isone in which the current drawn from the converter is larger than themaximum current that can be consistently delivered. Then the part of thedesired current that cannot be delivered will discharge the outputcapacitor of the supply, resulting in a decrease in output voltage—whichis equivalent to the output voltage at start-up being low. In start-upand over-current situations, as shown at the left-hand side of FIG. 2,if the output voltage (Vout) is too low to support operation of theoptocoupler, both the optocoupler input current and the optocoupleroutput (feedback) current below the nominal first maximum level.However, the controller would interpret this, that is to say, a lowfeedback current, as an indication that the voltage is too high, andthus that less power is required. As a consequence, the output voltagewill not be able to rise, and start-up will not properly occur.

One solution to this problem is to combine the inverted polarity gain asdescribed above with reference to FIG. 2, with a supplementarymeasurement of the output voltage made on the primary side (Voutprim).When Voutprim indicates that the upper voltage is below a certainthreshold level (V1), the inverted polarity gain is overridden. This isillustrated in FIG. 3. The figure shows, as FIG. 2, the feedback current10 plotted against output voltage. Above a reference voltage (Voutref)the feedback current falls with increasing voltage. However, thecontroller is only reliant on the feedback current for voltages 31 abovea threshold voltage (V1). For voltages 30 below the threshold, thefeedback loop is overridden. The threshold voltage is set sufficientlyhigh that the optocoupler is able to operate properly at this voltage.Below the threshold value, the converter operation is fully determinedby the primary side control, and can for example be set according to afixed value of a control parameter such as primary peak current, dutycycle, on-time or frequency, as will be known to those skilled in theart. In other embodiments, the setting for the control parameter iscombined with a time-varying element, for example, a slow increase overa certain time interval, in order to provide a soft start, or a valuewhich depends on the rate of rise of Voutprim, that is to say depends ondV/dT, to result in a well-defined output voltage rise during start-up.

An SMPS arrangement for using this method is shown in block diagram formin FIG. 4. FIG. 4 shows a power converter 41, which operates across themains isolation 42 between a primary side 43 and a second recite 44. Onthe secondary side there is a secondary side control block 45 a, whichreceives as input the output voltage (Vout). The secondary side controlblock 45 a drives the LED 46 a of an optocoupler 46. The photo diode 46p of optocoupler 46 feeds this information back to the power converter41. On the primary side there is the primary side control block 45 b,which provides a measurement (Voutprim) indicative of the outputvoltage, by means which will be immediately apparent to those skilled inthe art; for example by means of a primary side auxiliary transform orwinding. The measurement (Voutprim) indicative of the output voltage iscompared with a the threshold voltage V1 in a comparator 45 d, and theresult is supplied to the converter to determine whether control of theconverter is by means of the feedback current, or the measurement(Voutprim) indicative of the output voltage.

Use of the arrangement shown in FIG. 4 could result in an anomalous, orpotentially unsafe, situation where the input measurement circuit todetermine Voutprim malfunctions—for example, due to a broken or shortedconnection in the auxiliary winding circuit. The optocoupler may then bepermanently overridden, resulting in an uncontrolled output voltage.

To prevent this, in one embodiment, is there is provided an over-voltageprotection at the secondary side, which supplies additional informationto the primary side. This additional information may be sent via theoptocoupler by applying a current level outside the normal operationrange. Thus, if the output voltage exceeds an upper bound, both theinverted polarity feedback and the Voutprim measurement are overriddenby the abnormally high current on the optocoupler. Corrective actions,which may be applied when overvoltage is determined, may include acycle-by-cycle action, wherein the converter is disabled when the levelis above the over-voltage threshold but re-enabled when the level fallsbelow this threshold; alternatively a latch may be applied such that theconverter is disabled when the level is above the threshold and a saferestart is forced.

FIG. 5 shows a graph of the feedback current against output voltageaccording to this embodiment. The plot is similar to that shown in FIG.3; however in this case, where the output voltage exceeds an upper bound51, a abnormally high current 10 d is driven through the optocoupler'sLED 46 a. Since this abnormally high current 10 d exceeds an overvoltageprotection threshold value or upper bound 52, the controller is able torecognize that a malfunction has occurred. As shown, the abnormally highcurrent 10 d increases with yet further increase in output voltage,however, the skilled person will appreciate that the current may beinvariant with further increases in output voltage, or even fall withsuch increases. Furthermore, as shown in the figure, there may beincluded hysteresis 10 e around the upper bound 51, in order to preventbouncing between the normal-voltage, and overvoltage protected, states.

Another way of preventing the anomalous situation would be to provideextra timing information, which checks if the feedback current overridetakes a longer time than expected or desired, as an indication of amalfunction. If a malfunction is indicated, action can be taken, forinstance to disable the converter or disable the Voutprim measurementoverriding mechanism.

An arrangement for using the over-voltage protection method is shown inblock diagram form in FIG. 6. The figure is substantially the same asthat in FIG. 4, and like numerals are used to reference like components;however, in this case the secondary side control block 45 a includes anover-voltage protection control element 45 c

In another embodiment, the potential problem of a low output voltagebeing insufficient to drive the optocoupler is resolved by use of aminimum current threshold (Ithreshold). In this embodiment, arepresentation of the output current of the optocoupler is compared witha threshold, in order to define an overriding condition for theoptocoupler output. The control of the primary side is defined such thatthe current corresponding to the minimum required output power is largerthan the minimum value of the output current of the optocoupler, therebyensuring that the output can regulate to a minimum output power. As theoverriding condition for the optocoupler output is now based on theoptocoupler current, additional (primary side) sensing of output voltageis not necessary: absent any Voutprim measuring circuitry the aboveanomalous situation where such circuitry malfunctions is precluded.

A graph of the feedback current against output voltage for thisembodiment is shown in FIG. 7, and the corresponding block diagram inFIG. 8. In FIG. 7, similar to the previous embodiments, sections 10 band 10 a of the feedback current again form a monotonically decreasingfunction of voltage, with section 10 b being a flat and section 10 afalling linearly. However, in this embodiment, section 10 a does notfall to zero with increasing output voltage, but rather is clamped so asnot to be able to fall below a minimum value as shown at 10 f undernormal Vout conditions. This value is greater than that a predeterminedcurrent threshold level (Ithreshold). At low voltages, the current ismade to fall sharply at 10 c′, to zero—typically by switching off theoutput current from the error amplifier. Since this is less than thethreshold currents, the control is able to determine that the converteris operating in a low voltage regime.

The corresponding block diagram in use is shown in FIG. 8. Thisarrangement is similar to that shown in FIGS. 4 and 6; however, in thiscase the output current I_(Opto) from the optocoupler is also used toprovide an input into a comparator 81. Comparator 81 compares thiscurrent (I_(Opto)) with the threshold currents (I threshold), and if itis determined that the Opto current is below the threshold, theconverter is determined to be in a low voltage mode.

For the sake of completeness and the avoidance of doubt, it is herebyconfirmed that a “monotonically decreasing” function is a function whichdoes not increase; that is to say the function can either decrease orremain flat (i.e. neither increase nor decrease), or a combination ofdecrease and remain flat.

In summary, from one viewpoint, a controller for a Switched Mode PowerSupply having opto-coupler-based feedback from secondary to primaryside, has been disclosed, in which the optocoupler current variesinversely with the output voltage over a voltage control range. Theconverter is thereby enabled to consume less power than do conventionalconverters, when in lower-power standby mode. Also disclosed have beenlow-voltage startup and over-voltage protection arrangements combinedwith such a controller. Corresponding methods have been also disclosed.

From reading the present disclosure, other variations and modificationswill be apparent to the skilled person. Such variations andmodifications may involve equivalent and other features which arealready known in the art of power converters, and which may be usedinstead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations offeatures, it should be understood that the scope of the disclosure ofthe present invention also includes any novel feature or any novelcombination of features disclosed herein either explicitly or implicitlyor any generalisation thereof, whether or not it relates to the sameinvention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as does the presentinvention.

Features which are described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

The applicant hereby gives notice that new claims may be formulated tosuch features and/or combinations of such features during theprosecution of the present application or of any further applicationderived therefrom.

For the sake of completeness it is also stated that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, a single processor or other unit mayfulfil the functions of several means recited in the claims andreference signs in the claims shall not be construed as limiting thescope of the claims.

The invention claimed is:
 1. A controller configured to control aswitched mode power converter, the switched mode power converter havinga primary side, a secondary side isolated therefrom and for providing anoutput voltage, and an optocoupler therebetween for providing feedbackinformation to the primary side from the secondary side and drawing afeedback current from the secondary side, wherein the feedbackinformation is dependant on the output voltage (Vout) and the controlleris configured to provide that the feedback current (Iopto) monotonicallydecreases with increasing output voltage between a threshold voltage(V1) and at least an upper bound, characterised in that the controlleris configured such that the feedback current is invariant with theoutput voltage between the threshold voltage and a reference voltage(Voutref), which reference voltage is higher than the threshold voltage.2. A controller according to claim 1, wherein the controller isconfigured such that the feedback current either is inverselyproportional to the output voltage above the reference voltage (Voutref)or decreases linearly with increasing output voltage above the referencevoltage (Voutref).
 3. A controller according to claim 1, configured toreceive a signal (Voutprim) from the primary side indicative of theoutput voltage (Vout), and wherein the controller is further configuredto override the feedback information from the optocoupler with thesignal (Voutprim) when the output voltage is less than the thresholdvoltage.
 4. A controller according to claim 3, wherein the controller isconfigured to provide an over-voltage feedback current (10 d) in thecase that the output voltage is above the upper bound.
 5. A controlleraccording to claim 4, further comprising an over-voltage protectioncontrol element for triggering the over-voltage feedback current onlyafter a predetermined time has elapsed.
 6. A controller according claim1, wherein the controller is configured to provide a minimum feedbackcurrent, in the case that the output voltage is above the upper bound.7. A controller according to claim 6, wherein the controller isconfigured to override the feedback information from the optocoupler inthe case that the feedback information is less than a predeterminedthreshold current.
 8. A switched mode power converter comprising acontroller as claimed in claim
 1. 9. A method of controlling a switchedmode power converter having a primary side, a secondary side isolatedtherefrom and for providing an output voltage, and an optocouplertherebetween for providing feedback information and drawing a feedbackcurrent from the secondary side, the method comprising providingfeedback information to the primary side from the secondary sidedependant on the output voltage and generating the feedback current suchthat it monotonically decreases with increasing output voltage between athreshold voltage and at least an upper bound, and wherein the feedbackcurrent is invariant with the output voltage between the thresholdvoltage and a reference voltage (Voutref).
 10. The method of claim 9wherein the feedback current either varies in inverse proportion to theoutput voltage above the reference voltage (Voutref) or decreaseslinearly with increasing output voltage above the reference voltage(Voutref).
 11. The method of claim 9, further comprising measuring asignal (Voutprim) on the primary side indicative of the output voltage(Vout), and overriding the feedback information from the optocouplerwith the signal in the case that the output voltage is less than thethreshold voltage.
 12. The method of claim 11 wherein the controllerprovides an over-voltage feedback current in the case that the outputvoltage is above the upper bound.
 13. The method of claim 12, whereinthe over-voltage feedback current is triggered only once the outputvoltage has been above the upper bound for a predetermined period. 14.The method of claim 9, wherein the feedback current is set to a minimumvalue (10 f), in the case that the output voltage is above the upperbound.
 15. The method of claim 14, wherein the controller overrides thefeedback information from the optocoupler, in the case that the feedbackinformation is less than a predetermined threshold value which is lessthan the minimum current value (10 f).